1. Field of the Invention
The present invention is directed to a method for optimizing a charge image generation of a photoconductor of electrophotographic printer and copier devices.
2. Description of the Related Art
Compared to the copying results that can be achieved with electrophotographic copier devices, the user of such devices makes considerably higher quality demands of the printing results that can be achieved with electrographic printer devices. Users of copier devices therefore also accept copying results that are somewhat poorer compared to the original of the copy.
Since electrophotographic printer devices, however, are usually employed in conjunction with EDP (electronic data processing) systems and the possibilities for the user to influence the print quality are slight in this respect, extremely high quality demands exist in the case of electrophotographic printer devices. In order to do justice to these high demands, it is necessary to reduce the allowable tolerance ranges in electrophotographic processes.
Electrophotographic printer device print, for example, single sheets or continuous stock in that a latent image is generated on a photoconductor that preferably has the form of a drum. To this end, the photoconductor is charged to a defined charging potential. Subsequently, a latent image is generated on the photoconductor with an illumination means that supplies energy to the photoconductor in punctiform fashion, the latent image being generated in that the charge in the regions of the photoconductor is reduced to such an extent by illumination that these regions remains white in the subsequent printout in what is referred to as "charged area development" (CAD) or, respectively, are inked with toner in what is referred to as "discharged area development" (DAD). Following the exposure, toner is applied to the photoconductor with the assistance of a development means, the toner remaining adhering to the charged regions (CAD process) or, respectively, to the discharged regions (DAD process) of the photoconductor.
The toner image on the photoconductor is subsequently transferred onto, for example, paper or some other recording medium and is fused into the recording medium by heating in a following fixing station or connected thereto by adhesion forces arises when the toner image melts. After the toner image has been transferred onto the recording medium, the photoconductor is completely discharged and cleaned of residual toner in order to be subsequently completely charged again to a defined potential as preparation for the next exposure.
As FIG. 1 shows, the potential of the photoconductor upon illumination thereof decreases from a charging potential V2, to which the photoconductor was charge before the illumination and that can be kept constant with a charge regulation, to a substantially lower potential VD1 or, respectively, VD2 along a characteristic K1 or, respectively, K2. The potential of illuminated regions is dependent, on the one hand, on the height of the illumination energy, the duration of the illumination and the height of the charging potential V2.
On the other hand, the discharge characteristic K1, K2 and, thus, the height of the potential of exposed regions on the photoconductor is also influenced, for example, by manufacture-conditioned fluctuations, the quality of the photoconductor, its age, its temperature and is also influenced by the current process status just as, for example, the start of a printing process, longer pauses between individual printing processes or by a variety of environmental influences. Fluctuations in potential VD1, VD2 of the illuminated regions of the photoconductor derive therefrom that, due to the different toner acceptance in the development unit caused as a result thereof, lead to quality fluctuations of a print image to be produced.
For compensating temperature fluctuations, it is known to keep the photoconductor at a constant temperature during the entire operation. In part, the photoconductor is even kept at a constant temperature uninterrupted. What is thereby disadvantageous is that a corresponding heating must be provided, this causing increased energy costs. Moreover, only one of the aforementioned influencing factors is influenced by the heating.
It is also known to vary the toner concentration dependent on certain parameters for compensating quality fluctuations. However, all quality fluctuations cannot be compensated therewith. In particular, raster or fine-line reproductions cannot be kept constant in the same way by varying the toner concentration.
The Japanese Patent document JP 6-230642 (A) or, respectively, the appertaining Japanese Patent Application HEI sei 5-15327 discloses a method wherein, for optimizing the charge image generation, the discharge characteristic of the photoconductor is determined dependent on the illumination energy by repeated measurement of the discharge potential at different illumination energies and a subsequent approximation between the measured values. It is disadvantageous that both a plurality of measurements are required and that an approximation must also be subsequently made in order to determine the optimum value for the illumination energy.
U.S. Pat. No. 4,855,766 explains a method for determining an optimum charging potential and an optimum illumination energy, whereby the charging potential or, respectively, the illumination energy is incremented step-by-step by a predetermined amount until a rated value is reached. In this method, thus, a plurality of iteration steps are to be implemented as a rule until the rated value for the difference of the discharge potential and the charging potential or, respectively, the rated value for the rise of the discharge curve of the photoconductor given an optimum illumination energy is reached.
German Published Application 27 41 713 discloses a method for the stabilization of a charge image wherein, given a predetermined illumination energy, the optimum charging potential can in fact be determined with one or two iteration steps but coefficients of functions must be previously determined that were derived from a model of the photoconductor and, accordingly, take a plurality of influencing quantities into consideration. As a result thereof, however, the equations become very complex and the calculating time for the calculation of the coefficients rises. Ultimately, six value groups of the charging potential and of the discharge potential must be measured until the sought coefficients can then be determined by solving an equation system. However, the coefficients can then still have great errors. Alternatively, the registration of a plurality of measured values is respectively proposed for the determination of one of the coefficients, from which the sought coefficients are then determined with the required precision with the methods known from linear optimization.
U.S. Pat. No. 4,502,777 discloses a plurality of physical relationships in a copying process and, building thereon, discloses a comparatively complicated method for the correction of the charging voltage or, respectively, of the current flowing through the charging unit that is implemented without measuring the charging potential or the discharge potential. When explaining the physical relationships, an iteration method for determining the charging potential or, respectively, the current flowing through the charging device given a constant illumination energy is also recited. In addition to the iteration steps to be multiply implemented, however, this iteration method has the disadvantage that it assumes an approximately linear relationship between discharge potential and illumination energy.